Field effect transistors (FETs) may be used as sensors by utilising the gate electrode of the FET as a sensor electrode. The gate electrode may be coated with or formed from a substance that can interact with a fluid analyte such that the operation of the FET becomes sensitive to, for example, chemical compounds, particles or ions contained in the analyte. Such sensors have applications as pH sensors. In applications the electrical current through FET based sensors may be dependent on the concentration of the chemical compound in the analyte when the analyte is in contact with the gate electrode of the FET.
One particular advantage of using FET based sensors is that they may be easily integrated into an integrated circuit package, such that the signal processing components associated with, and/or ancillary to the sensor can be provided on the same semiconductor die, thus yielding a compact sensor arrangement and lower manufacturing costs. Furthermore, by integrating sensors into an integrated circuit package processing of the entire sensor integrated package can be simplified.
However, for FET based sensors it may be necessary to separate the gate electrode from the analyte such that the gate electrode is not in contact with the analyte. Separating the gate electrode from the analyte prevents any contact between the analyte and the gate electrode which could result in chemical reactions between the analyte and gate electrode irreparably damaging the FET.
One particular arrangement of a FET based sensor which separates the gate electrode and analyte utilises an extended gate electrode FET (EGFET) arrangement. The EGFET structure may comprise a gate electrode, which is electrically connected to a measurement electrode, and a reference electrode. The measurement electrode and reference electrode may be in contact with the analyte. The EGFET will begin to exhibit a current flow through the FET when the potential difference between the gate electrode and the reference electrode, caused by the analyte, exceeds a threshold voltage of the EGFET. The threshold voltage is the voltage at or near which the EGFET begins to conduct.
However, one problem with the known EGFET arrangements is that the potential difference between the measurement electrode and the reference electrode, caused by the analyte, may not be greater than the threshold voltage of the FET due to, for example, low concentrations of chemical compounds in the analyte. By applying a bias voltage to the gate electrode and the reference electrode it may be possible to lower the threshold voltage of the FET. However, the bias voltage may cause an unwanted electrical current to flow through the analyte causing chemical reactions in the fluid analyte which may result in sensor drift and/or corrosion of the measurement electrode and/or the reference electrode which can adversely affect the predictability and accuracy of the sensor.